The subject matter disclosed herein relates generally to multiple alloy turbine rotors, and more particularly to multiple alloy turbine rotors having a plurality of welded rotor sections.
The operating temperature of turbine rotors varies along the length of the rotor, with the first stage having the highest temperature, with the subsequent stages generally having progressively lower temperatures. The different operating conditions complicate the selection of a suitable rotor material and the manufacturing of the rotor because a monolithic rotor (i.e., a rotor that is not an assembly) of a single chemistry cannot meet the property requirements of each of the low pressure (LP), intermediate pressure (IP) and high pressure (HP) sections or stages of the turbine. For example, the inlet and exhaust areas of a turbine rotor, such as a turbine rotor for an industrial gas turbine, have different material property requirements. The high temperature inlet region typically requires a material with high creep rupture strength but only moderate toughness. The exhaust area, on the other hand, does not demand the same level of high temperature creep strength, but suitable materials typically must have very high toughness because of the higher loads imposed by longer turbine blades used in the exhaust area. In order to tailor the properties of the rotor and limit utilization of higher cost, high temperature, high strength alloys to only the portions of the rotor where they are needed, and to ensure high toughness and other properties where they are needed, various approaches have been utilized.
For the reasons discussed above, rotors constructed by assembling sections of different chemistries are widely used. Rotors for gas turbines and jet engines are often constructed by bolting a series of disks and shafts together. For example, large steam turbines typically have a bolted construction made up of separate rotors contained in separate shells or hoods for use in different sections of the turbine. Smaller steam turbines may make use of a mid-span coupling to bolt high and low temperature components together within one shell. While rotors having a bolted construction are widely used, they suffer from several disadvantages including increased numbers of parts, increased assembly requirements, increased length of the rotor assembly, and increased complexity associated with achieving the necessary balance of the rotor assembly.
One piece or monolithic multiple alloy turbine rotors have been developed to provide high temperature, high strength alloys where they are needed, such as in the HP and IP sections of the rotors, and to utilize lower cost, lower strength, high toughness alloys in the cooler portions of the rotors, such as the LP section. While monolithic multiple alloy rotors are known, the materials and processes needed to manufacture them as large rotor forgings are complex and costly. Further, replacement of a particular rotor section is generally not possible, which also is undesirable from the standpoint of ease and cost of maintenance of the turbine over its operating lifetime.
Multi-section, multiple alloy rotors made by welding dissimilar metal alloys have also been proposed; however, their use has been limited due to one or more of the following concerns typically associated with dissimilar alloy weld joints. One concern is high weld cracking susceptibility due to intermixing of widely different chemistries in the weld pool that result in solidification over a wide temperature range, which can in turn correspond to a wide range of melting points within the weld. Another concern is heat-affected zone cracking from mechanisms such as intergranular liquidation caused by low melting temperature phases, such as eutectic phases, or strain age cracking. Still another concern is poor weld joint mechanical properties, such as tensile strength, ductility, high cycle and low cycle fatigue, creep rupture, fracture toughness and the like, due to the formation of complex phases from the intermixing of alloys having widely different chemistries. Still another concern is high transient thermal strains due to thermal expansion mismatch across the weld joint. Another concern is the potential for long-term microstructural instability in high temperature operation due to complex metastable phases in the weld joint and diffusion effects that can result in the formation of brittle phases in the weld joint, such as various intermetallic phases. Yet another concern is the segregation of carbon, boron, and other elements across the weld diffusion zone either during post weld heat treatment or during long term service. Such segregation is caused by variation in the chemistry between the weld and the parent metal. Such effect can cause degradation of critical properties and cracking susceptibility.
Past approaches for welding of dissimilar alloys to form turbine rotors have involved the buildup of fusion welded clad layers of various chemistries on the joint face of one or both rotor sections. The clad layers have included those having uniform or varying alloy chemistries. The cladding is heat treated and machined prior to welding the preforms. This approach is costly, time consuming and also may not alleviate some of the concerns described above related to the welding of dissimilar metals.
While various multiple alloy welded turbine rotor configurations and methods for their manufacture are known, all known constructions involve welding of dissimilar metals and are subject, in varying degrees, to the concerns related thereto described above.
Therefore, welded multiple alloy rotor configurations and methods of their manufacture that reduce or eliminate concerns associated with the welding of dissimilar metals are desirable.